查詢數(shù)據(jù)指從數(shù)據(jù)庫中獲取所需要的數(shù)據(jù)。查詢數(shù)據(jù)是數(shù)據(jù)庫操作中最常用，也是最重要的操作。用戶可以根據(jù)自己對數(shù)據(jù)的需求，使用不同的查詢方式。通過不同的查詢方式，可以獲得不同的數(shù)據(jù)。MySQL中是使用SELECT語句來查詢數(shù)據(jù)的。在這一章中將講解的內(nèi)容包括。
1、查詢語句的基本語法
2、在單表上查詢數(shù)據(jù)
3、使用聚合函數(shù)查詢數(shù)據(jù)
4、多表上聯(lián)合查詢
5、子查詢
6、合并查詢結(jié)果
7、為表和字段取別名
8、使用正則表達(dá)式查詢
什么是查詢？
怎么查的？
數(shù)據(jù)的準(zhǔn)備如下：
| create table STUDENT( STU_ID int primary KEY, STU_NAME char(10) not null, STU_AGE smallint unsigned not null, STU_SEX char(2) not null ); insert into STUDENT values(2001,'小王',13,'男'); insert into STUDENT values(2002,'明明',12,'男'); insert into STUDENT values(2003,'紅紅',14,'女'); insert into STUDENT values(2004,'小花',13,'女'); insert into STUDENT values(2005,'天兒',15,'男'); insert into STUDENT values(2006,'阿獵',13,'女'); insert into STUDENT values(2007,'阿貓',16,'男'); insert into STUDENT values(2008,'阿狗',17,'男'); insert into STUDENT values(2009,'黑子',14,'男'); insert into STUDENT values(2010,'小玉',13,'女'); insert into STUDENT values(2011,'頭頭',13,'女'); insert into STUDENT values(2012,'冰冰',14,'女'); insert into STUDENT values(2013,'美麗',13,'女'); insert into STUDENT values(2014,'神樂',12,'男'); insert into STUDENT values(2015,'天五',13,'男'); insert into STUDENT values(2016,'小三',11,'男'); insert into STUDENT values(2017,'阿張',13,'男'); insert into STUDENT values(2018,'阿杰',13,'男'); insert into STUDENT values(2019,'阿寶',13,'女'); insert into STUDENT values(2020,'大王',14,'男'); |
然后這是學(xué)生成績表，其中定義了外鍵約束
| create table GRADE( STU_ID INT NOT NULL, STU_SCORE INT, foreign key(STU_ID) references STUDENT(STU_ID) ); insert into GRADE values(2001,90); insert into GRADE values(2002,89); insert into GRADE values(2003,67); insert into GRADE values(2004,78); insert into GRADE values(2005,89); insert into GRADE values(2006,78); insert into GRADE values(2007,99); insert into GRADE values(2008,87); insert into GRADE values(2009,70); insert into GRADE values(2010,71); insert into GRADE values(2011,56); insert into GRADE values(2012,85); insert into GRADE values(2013,65); insert into GRADE values(2014,66); insert into GRADE values(2015,77); insert into GRADE values(2016,79); insert into GRADE values(2017,82); insert into GRADE values(2018,88); insert into GRADE values(2019,NULL); insert into GRADE values(2020,NULL); |
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